Resolving the "'React' refers to a UMD Global, but the Current File is a Module" Error in TypeScript Projects

• UMD Global: UMD (Universal Module Definition) is a way to package JavaScript libraries so they can work in various environments, including browsers (as globals) and module systems like CommonJS or ES Modules. When you include a UMD library like React (before version 17), it becomes a global variable accessible throughout your code.
• Modules: In modern JavaScript, modules are the preferred way to organize code. Modules allow for better code organization, dependency management, and prevent naming conflicts. TypeScript, a superset of JavaScript that adds static typing, also uses modules.

The Conflict:

The error arises when you're using TypeScript with React (especially versions before 17) in a module system. TypeScript expects you to import React explicitly using an import statement, but if you're including it as a UMD global, it's already defined as a global variable. This creates a conflict because TypeScript doesn't know how to handle both at the same time.

Resolving the Error:

There are two main ways to fix this error:

1. Import React:

   • If you want to use React within a module system, the recommended approach is to import it using the import statement:

   import React from 'react';
   
   function MyComponent() {
       return <div>Hello, world!</div>;
   }
   

2. Configure TypeScript's JSX Transformation (for React versions before 17):

   {
       "compilerOptions": {
           "jsx": "preserve" // Or "react"
       }
   }
   

   This tells TypeScript to leave JSX syntax alone and not expect an imported React object.

Choosing the Right Approach:

• For Modern Projects: Importing React is generally considered the better practice for modern JavaScript projects that use modules. It improves code organization, maintainability, and reduces the likelihood of naming conflicts.
• For Older Projects (with React < 17): If you're working with an older project that uses React as a global variable, you can use the TypeScript JSX transformation configuration as a temporary solution. However, consider migrating to an import-based approach for long-term maintainability.

// myComponent.tsx
import React from 'react'; // Import React explicitly

function MyComponent() {
  return <div>Hello, world!</div>;
}

export default MyComponent; // Export the component

Explanation:

• We import React from the 'react' module using the import statement.
• Now, React is a local variable within this module, and TypeScript can understand its usage within JSX syntax.

a) tsconfig.json Configuration:

{
  "compilerOptions": {
    "jsx": "preserve" // Or "jsx": "react" (both work here)
  }
}

b) Code using React as a Global Variable:

// myComponent.tsx (assuming React is included as a UMD global)

function MyComponent() {
  return <div>Hello, world!</div>;
}

export default MyComponent;

• We don't explicitly import React here.
• The tsconfig.json configuration tells TypeScript to treat JSX syntax as plain JavaScript (without expecting an imported React object).

• Use approach 1 (importing) for new projects and projects transitioning to modern JavaScript practices.
• Consider approach 2 (configuration) only for older projects using React as a global variable and if updating to an import-based approach is not immediately feasible.

• If you're using a build tool like Webpack or Rollup, you can configure it to automatically handle UMD libraries like React (before version 17). These tools can bundle the UMD library and provide it as a CommonJS or ES module, eliminating the need for manual configuration in tsconfig.json.

Type Definitions for UMD Globals (Less Common):

• In rare cases, you might find third-party type definitions for UMD libraries like React. These definitions can provide type information to TypeScript even when using the library as a global variable. However, this approach is less common and requires finding compatible type definitions for your specific version of React.

Important Considerations:

• Legacy Projects: If you're working with a legacy project with React as a global variable, consider using the TypeScript JSX transformation configuration or a build tool with bundling as temporary solutions. Aim to migrate to an import-based approach for long-term maintainability.
• Third-Party Libraries: Be cautious of relying on third-party type definitions for UMD globals. Ensure they are well-maintained and compatible with your project's requirements.